A wireless communication eliminates the burden of wiring operations in a traditional wired communication and further serves for a utilization as a technology for realizing a mobile communication. For example, as a regular standard with regard to a wireless LAN (Local Area Network), IEEE (The Institute of Electrical and Electronics Engineers) 802.11 can be exemplified. IEEE802.11a/g has been already widely spread.
In many wireless LAN systems including IEEE802.11, while an access control procedure based on a carrier sense such as CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) is adopted, respective communication stations are set to avoid collisions of carriers at a time of random channel access. That is, a communication station in which a transmission request is generated first monitors a media state for a predetermined inter frame space DIFS (Distributed Inter Frame Space), performs random back-off if a transmission signal does not exist during this period, and obtains a transmission right and can transmit a frame in a case where the transmission signal does not exist further during this period too. Also, when a frame with a high degree of urgency such as ACK is to be exceptionally transmitted, the communication station is allowed to transmit a frame after a shorter inter frame space SIFS (Short Inter Frame Space). According to this, the frame with the high degree of urgency can be transmitted ahead of a frame that is transmitted in accordance with a normal CSMA procedure.
Also, in the wireless communication, it is known that a hidden terminal problem occurs in which an area exists where the communication stations cannot mutually communicate directly. As mutual hidden terminals cannot perform a negotiation, a possibility exists that transmission operations may collide with each other. As a methodology for solving the hidden terminal problem, “virtual carrier sense” can be exemplified. To be specific, in a case where Duration (duration time) information for reserving media is described in a reception frame whose destination is not the local station, the communication station expects that the media is used during a period in accordance with the Duration information, that is, performs the virtual carrier sense and sets a transmission stop period (NAV: Network Allocation Vector).
As a representative example of a signal transmission reception sequence utilizing the virtual carrier sense, RTS/CTS handshake can be exemplified. A communication station at a data transmission source transmits a transmission request frame (RTS: Request To Send), and a data transmission is started in response to a reception of a confirmation notification frame (CTS: Clear To Send) from a data transmission destination. Then, when the hidden terminal receives at least one of the frames RTS and CTS whose destination is not the local station, the transmission stop period is set on the basis of the Duration information described in the reception frame to avoid the collisions. By using the RTS/CTS handshake in combination with the CSMA/CA control procedure, reduction in overhead of the collisions in an overloaded state may be realized in some cases.
Also, according to the standard of IEEE802.11a/g, in a 2.4 GHz band or 5 GHz band frequency, by utilizing an orthogonal frequency division multiplexing (Orthogonal Frequency Division Multiplexing: OFDM), a modulation method that achieves a communication speed of 54 Mbps at maximum (physical layer data rate) is supported. In IEEE802.11n that is an extended standard, a further higher bit rate is realized by adopting an MIMO (Multi-Input Multi-Output) communication system. Herein, MIMO refers to a communication system provided with a plurality of antenna elements on both a transmitter side and a receiver side for realizing spatially multiplexed streams (widely known).
IEEE802.11n maintains a backward compatibility with IEEE802.11a/g. For example, in a frame format of IEEE802.11n, a method of spoofing a signal information (L-SIG) field where a header section guarantees the backward compatibility is adopted. To be specific, spoofed frame length information and transmission rate information are described in the signal information (L-SIG), and a legacy communication terminal compliant with IEEE802.11a/g is caused to recognize that the frames continue during a period until a frame exchange sequence having no backward compatibility is completed and stand by for a transmission operation to avoid the collisions. On the other hand, with respect to a communication terminal compliant with IEEE802.11n, it is indicated that L-SIG is spoofed by switching a signal arrangement of a part of a signal information (HT-SIG) field where the backward compatibility of the header section is not guaranteed. Therefore, a high speed communication terminal compliant with IEEE802.11n obtains correct Duration information based on a decoding result of HT-SIG and can perform an appropriate virtual carrier sense (for example, see PTL 1).
Although a high throughput (High Throughput: HT) above 100 Mbps can be achieved by IEEE802.11n, along with an increase in the information amount of transmission contents, realization of a further higher speed is demanded. For example, as the number of antennas in the MIMO communication device is increased and the number of streams to be spatially multiplexed is increased, it is possible to improve the throughput in a one-to-one communication while the backward compatibility is maintained.
In future, an improvement in the throughput for the plurality of users as a whole is demanded in addition to the throughput per user in the communication. For example, the working group for IEEE802.11ac aims to establish a wireless LAN standard in which a frequency band smaller than or equal to 6 GHz is used and a data transmission speed exceeds 1 Gbps, and for the realization, like multi user MIMO (MU-MIMO) or SDMA (Space Division Multiple Access), a space division multiple access system where a wireless resource on a spatial axis is shared by a plurality of users is potent.
At present, the space division multiple access is under review as one of fundamental technologies for a next generation mobile phone series system based on a time division multiple access (Time Division Multiple Access: TDMA) such as PHS (Personal Handyphone System) or LTE (Long Term Evolution). Also, in a wireless LAN field, a one-to-many communication is being paid attention as described above, but an application example is rarely met. This is probably because it is difficult to efficiently multiplex the plurality of users in the frame communication.
Also, when an operation of the space division multiple access is started by the new wireless LAN standard, as a communication device of the relevant new standard needs to operate under a communication environment where a communication device of the conventional standard exists in a mixed manner, it is necessary to sufficiently take the backward compatibility with the conventional standard into account.
For example, when the communication device compliant with the new standard transmits frames to a plurality of communication partners at the same time by applying the space division multiple access, the hidden terminal that contains at least one of the communication stations at the transmission source and the transmission destination of the relevant frame in a communication range refrains from a transmission operation over a period until the series of frame exchange sequence is ended even when a multiplexed signal cannot be decoded as not being compliant with the new standard and needs to avoid collisions of transmission signals.
According to the conventional IEEE802.11, a mechanism of the carrier sense such as CSMA/CA and RTS/CTS is introduced. Therefore, in the new standard such as IEEE802.11ac, it is necessary to preferably combine the carrier sense with the space division multiple access.
For example, a communication system is proposed in which two technologies of the carrier sense in the conventional IEEE802.11 and the space division multiple access by an adaptive array antenna are combined with each other by using the RTS, CTS, and ACK frames composed of a frame format that maintains backward compatibility with the conventional IEEE802.11 (for example, see PTL 2).
Also, the communication station can perform the space division multiple access as the plurality of antenna elements function as the adaptive array antenna, but for that, it is necessary to previously perform the learning on the weight of the adaptive array antenna. For example, the communication station can learn the weight of the adaptive array antenna by obtaining it from the training signal that is received from each of the plurality of communication partners. Alternatively, by using a predetermined adaptation algorithm such as RLS (Recursive Least Square) with respect to the training signal, it is possible to directly carry out the learning on the weight of the adaptive array antenna (for example, see PTL 2).
In either method, the communication station that performs the learning on the weight of the adaptive array antenna needs each of the communication partners to send the training signal. Also, under the communication environment where the communication apparatuses that only follow the conventional standard exist in a mixed manner, similarly as in a state in which the normal frame exchange sequence needs to be carried out while avoiding the collisions of the carriers, the training signal needs to be transmitted while avoiding the interference by the communication apparatuses that only follow the conventional standard. That is, it is necessary to learn the weight of the adaptive array antenna while the backward compatibility with the conventional standard is maintained.